Living organisms experience daily, seasonal, and long-term climatic variations in temperature. Many animals respond to changes in body temperature (Tb) with behavioral adjustments, migration, or changes in metabolism (e.g., torpor). However, for some animals and almost all plants such responses are impossible. Animals that effectively regulate their Tb to a thermal set-point despite larger variations in ambient temperature are referred to as homeotherms (in Greek homoios means 'same, identical', and therm means 'heat'). Stenotherms (in Greek Stenos means 'narrow', and therm means 'heat') are animals that live in a thermally stable environment and can tolerate only narrow temperature changes. Excellent examples of stenotherms are the Antarctic marine notothenioid fishes and invertebrates that have evolved in waters having an annual temperature variation of less than 2 °C. However, despite the narrow range in Tb of stenotherms, they are not homeotherms as they do not actively regulate Tb. The regulation of a Tb in homeotherms, independent of ambient temperature, is determined by the rates of heat gain and/or loss through a combination of physical and physiological means and, in some cases, by heat production.

Although not necessary for homeothermy, many homeotherms regulate Tb through the retention of meta-bolically generated heat, that is, endothermy (in Greek endo means 'within' and therm means 'heat'). Examples of endothermy have arisen in some plants, insects, fishes, and reptiles, and in all mammals and birds (Figure 1). Many endotherms (e.g., hummingbirds, echidnas, bill-fishes) show substantial daily or seasonal variation in Tb, which in many cases is associated with variation in the thermal set-point. Among ectotherms (in Greek ecto means 'outside' and therm means 'heat') there are examples where homeothermy is achieved through heat gained from the environment and where behavioral thermoregulation coupled with improved heat retention enables thermal stability. What then are the benefits and costs of homeothermy? To appreciate the importance of home-othermy in deciding the metabolic niche space of an animal one needs to consider how metabolic rate and other physical factors such as temperature and body size determine an animal's Tb and subsequent life functions.

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